Many schemes have been developed to protect data from accidental loss or damage. One of them is hardware redundancy schemes, such as redundant arrays of independent disks (RAID).
Unfortunately, hardware redundancy schemes are ineffective in dealing with logical data loss or corruption. For example, a file deletion or virus infection is often automatically replicated to all of the redundant hardware components and can neither be prevented nor recovered from by such technologies.
To overcome this problem, backup technologies have been developed to retain multiple versions of a production system over time. This has allowed administrators to restore previous versions of data and to recover from data corruption.
One type of data protection system involves making point in time (PIT) copies of data. A first type of PIT copy is a hardware-based PIT copy, which is a mirror of a primary volume onto a secondary volume. The main drawbacks of the hardware-based PIT copy are that the data ages quickly and that each copy takes up as much disk space as the primary volume. A software-based PIT, so called a “snapshot,” is a “picture” of a volume at the block level or a file system at the operating system level.
Backup data is generated in accordance with a data backup policy. Typically, the data backup policy sets an expiration time of each backup. For example, a system may retain all writes to the system for two days to provide any-point-in-time protection, and retain hourly snapshots for two weeks, daily snapshots for two months, and monthly snapshots for one year. Each snapshot has its own expiration time. Typically, the expiration time is determined by a main system clock. The system automatically deletes backup data upon expiration of the timer of each backup in accordance with the main system clock.
If a system operator accidentally or maliciously advances the main system clock, the system would automatically delete snapshots or a metadata timer of which is set before the accidentally or maliciously advanced time. In that situation, the system may or may not recover the deleted data.